An attempt has been made to identify and study by hybridoma technology a library of new antigens specific for tissues with neuroendocrine function such as hypothalamus, pituitary, and placental tissues. Mice were immunized with polymerized antigens prepared from these tissues. Hybridoma clones producing antibodies that reacted with antigens in the tissue were established on the basis of ELISA and RIA. Identification of each antigenic molecule can be achieved by the immunoblot technique. Localization of antigens in tissue has been studied by immunocytochemical evaluation. Monoclonal antibodies specific for neurophypsin antigens in hypothalamus and posterior pituitary are expected to help dissect the molecular heterogeneity of neurophypsins.